The present invention relates to a process for fractionating water-soluble or water-dispersible amino-containing polymers having a broad molar mass distribution by ultrafiltration, and the use of the polymer contained in the retentate.
Water-soluble amino-containing polymers have long been used as retention aids, drainage aids and fixing compositions in papermaking, as promoters in the sizing of paper with alkyldiketenes, as flocculants for sewage sludges, as adhesion promoters in the production of laminated films, as additives in hair setting and skincare compositions and as compositions for immobilizing anionic active ingredients.
As a rule, such amino-containing polymers are adducts and/or condensates of amino-containing building blocks, such as alkyleneimines, diamines and oligoamines, which may have been reacted with carboxylic acids, carboxylic acid derivatives, polyethers, polyesters and/or crosslinking agents, cf. for example DE-B-1 771 814, U.S. Pat. No. 2,182,306, U.S. Pat. No. 3,203,910, U.S. Pat. No. 4,144,123, EP-A-0 411 400, DE-A-2 162 567, U.S. Pat. No. 4,066,494, DE-A-2 916 356, WO-A-94/12560, WO-A-94/14873 and Journal of Applied Polymer Science, Vol. 30, 4099-4111 (1984).
WO 97/25367 describes a process for preparing water-soluble, amino-containing condensates and adducts by ultrafiltration of aqueous solutions of the condensates or adducts through membranes, from 5 to 95% by weight of the condensates or adducts contained being separated off as permeate. The amino-containing condensates and adducts obtained by this process have a comparatively narrow molar mass distribution and very good performance characteristics as retention aids, drainage aids and fixing compositions in papermaking. The disadvantage of these processes is that the amino-containing adducts and condensates obtained have very high Brookfield viscosity of up to 850 mPas, based on a roughly 15% strength by weight polymer solution, which has an adverse effect on the ultrafiltration process.
EP-B-0 442 866 describes a process for separating and reusing urea/formaldehyde, melamine/formaldehyde and polyamidoamine/epichlorohydrin resins by ultrafiltration. This process is intended to prepare polymers having a high molecular weight and to reduce the concentration of pollutants, such as formaldehyde. The permeate separated off can be worked up, if necessary, before being recycled to the polymer preparation. Also described is a plant for the process, which comprises a polymerization reactor, an ultrafiltration unit and a permeate working-up stage. The ultrafiltration described is a batch procedure, some of the polymer solution being removed from a storage container, subjected to ultrafiltration and then recycled to the container, and the process is repeated until the pollutant content has been reduced to the desired extent or until the desired concentration of the polymer has been reached. The possibility of a continuous procedure is merely indicated, and no further information is given.
The disadvantage of the batchwise process for the ultrafiltration of polymers is that, in order to achieve sufficient separation, the retentate has to be recycled again and again to the product tank until the desired concentration has been reached. After sufficient separation, the product tank must be emptied and filled with fresh starting material and the ultrafiltration must be started and run in again. This leads to downtimes during filling and emptying and to varying product qualities from filtration to filtration. To avoid the downtimes during filling and emptying, a batchwise plant can be operated cyclically by means of two product tanks, i.e. while the ultrafiltration is being carried out from one tank the other tank can be emptied and refilled. However, the additional plants for cyclic operation acquire a large amount of space and are expensive. To ensure a very uniform quality of the separation products, the filter units would have to be cleaned and/or maintained regularly in the case of batchwise operation, i.e. as a rule after every batch. Another disadvantage is the large viscosity and pressure increase toward the end of the ultrafiltration, which results from the increasing concentration of the retentate and requires efficient conveying means and pressure-resistant plants. Furthermore, in the case of high viscosities, either the transmembrane pressure must be increased or the inflow reduced. Both may adversely affect the separation efficiency.
In Milk Science International 36 (11) 1981, J. Hiddink, R. De Boer and P. F. C. Nooy report on investigations into transmembrane flow in the ultrafiltration of whey. The effect of a thermal pretreatment, of pH, of the salt content and of the membrane type used is investigated. A batchwise and continuous ultrafiltration plant were used for the investigation, and no dependence of the results on the continuous and batchwise procedure was found.
It is an object of the present invention to provide a process for fractionating at least partly water-soluble, amino-containing synthetic polymers, which is simple to carry out in practise and leads to an amino-containing polymer having a low viscosity and having at least comparable performance characteristics, such as a good drainage and retention action, in papermaking.
We have found that this object is achieved, according to the invention, by a process for the fractionation of water-soluble or water-dispersible amino-containing synthetic polymers having a broad molar mass distribution by ultrafiltration, wherein the polymer solution or dispersion to be fractionated is fed continuously into at least one ultrafiltration circulation with at least one ultrafiltration unit, and retentate having a narrower molar mass distribution and permeate are discharged continuously, in such a way that the ultrafiltration circulation is essentially in a steady state.
We have surprisingly found that the amino-containing polymers obtained by this process have very good performance characteristics in combination with a lower viscosity. While the polymer transmembrane flow decreases in the batch process at high concentration, in the continuous process a higher polymer, transmembrane flow is surprisingly observed on going over to higher concentrations. Consequently, in the novel process, the relative requirement with respect to membrane area and delivery decreases substantially in comparison with the batch process if the procedure is carried out at high concentration, although the final concentration of the polymer is constant. This makes it possible to save filtration water, and permeates of higher concentration are obtained. As a result, the cost of any required reconcentration of the permeate can be substantially reduced.